To increase the efficiency of combustion systems and at the same time to reduce the emission of pollutants from gas turbine engines and furnaces, various improvements for burners have been proposed in the past. Moreover, legal requirements on maximum emission limits have gone into effect which must be met by the combustion systems. Aside from other combustion products, hydrocarbons and nitrogen oxides (NOx) are responsible for air pollution.
Acting as the nitrogen sources in this connection are, on the one hand, combustion air in the form of ordinary ambient air with its natural nitrogen content, and on the other hand, fuel, containing organically bonded nitrogen. In the combustion itself, the formation of nitrogen oxides depends heavily, among other things, on the length of time spent by the molecular nitrogen in the flame region. The shorter the time spent, the fewer the nitrogen oxides formed. Reducing the length of time, however, for instance by higher air or fuel flow rates, results in more noncombusted fluid compounds appearing in the emissions of the burner, causing a reduction in its efficiency.
The formation of carbon monoxide goes hand in hand with the formation of soot. Carbon monoxide has a high thermic value, which is thus lost for purposes of usable combustion.
Furthermore, the formation of nitrogen oxide is dependent on the flame temperature and increases with a rise in flame temperature. On the other hand, an increased flame temperature is desired to obtain a better fuel energy yield.
In a known burner for an airplane jet engine (DE-OS 30 17 034), in order to achieve both low emission of pollutants with fewer nitrogen oxides and increased efficiency, the burner head thereof is equipped with a concentric outlet arrangement in the form of several concentric, individually switchable outlet nozzles, with the fuel and air outlet nozzles alternating radially with each other starting from the center of the burner head. Depending on what single outlet nozzle is switched on, which depends in turn on the operating conditions, one (for idling) or two (for full load) combustion zones result. Hard-to-burn, noncombusted gas compounds are expelled with the emissions of this known burner and thus substantially lower the energy yield of the fuel.
Aside from their use in engines, burners are also used in furnaces. Here as well, various past improvements have been able to contribute to saving energy and reducing the emission of pollutants. One known method of reducing pollutants is that of external smoke gas recirculation. Ordinarily, in this method the smoke gas developed during combustion is returned to the combustion zone via external recirculation conduits partly with additional blowers. Unfortunately, this is accompanied by a lowering of the flame temperature, which decreases the nitrogen oxide formation.
Various methods of smoke gas recirculation are proposed in the publication "Technische Dokumentation Saacke," 1st edition 3/1990, of Saacke GmbH.
In the same publication an internal smoke gas recirculation is also described. Smoke gas herein is returned from the region after the flame end to the flame root by suitable means built into a combustion chamber, in the form of a pipe spaced apart from the outer wall of the combustion chamber. To be sure, with this internal smoke gas recirculation the emission of pollutants is also reduced by impeding the formation of nitrogen oxide; however, in this case as well a lowering of the flame temperature must also be tolerated, whereby the energy yield of the fuel is diminished.
A burner having a means for recirculation of combustion products is known from DE 40 20 237 A1. This known burner is specially designed in such a way that particularly the proportion of CO in the exhaust fumes is reduced. For this purpose, combustion air is fed to the marginal zone of the combustible gases, in order to attain a more complete combustion of the fuel in the flame tube and a corresponding reduction in the CO values. To be sure, the proportion of nitrogen oxides is possibly also reduced by this injection of air; however, in any case the combustion temperature is lowered, something which--as in the other known burners described above--entails the disadvantage that the fuel energy yield is thereby lowered. The return of combustible gases from the flame tube via the recirculation device into a mixing tube forming the chamber located in the outlet housing cannot effect an increase in the flame temperature, as the combustion zone does not extend into the mixing tube and the apertures through which combustion air is fed to the marginal zone of the combustible gases are not located too close to the entry end of the recirculation device in order that no additional air can get into the backflowing gases.